87 GN AC Compressor Ground Location ?

[Agent13]

A while back I converted my brakes to Vacuum from PM. After the conversion the AC would not turn on. I have power to the compressor, but missing continuity on the ground circuit. I momentarily installed a jumper to ground, the compressor started and the system blew cold air, as well as controlled on and off.
Question is, where is the ground point for the AC compressor plug ?
I can easily install another ground to the compressor plug, but would rather have the system wired the way it is supposed to be. Not sure if I knocked off the ground wire when replacing the PM or not, don't see any extra or loose wires hanging around.
The wiring diagram shows the negative wire just going to ground from the compressor so installing another ground should not cause an issue ?
Any help would be appreciated.

 

[chopped39]

Alldata shows the black wire going to the high pressure cut out and then it grounds at the right rear cylinder head.It says its the same ground for the fan. So if your fan works you have an open ground. hope this helps
Adding another ground is ok but I always like to know where the problem is at. but make sure you ground it at the high pressure switch. You dont want the high side building to much pressure

 

[Agent13]

Thanks for the reply. I have checked the high pressure switch and the plug. The cooling fan high speed can be forced on from the plug and the switch side is closed on the AC Comp clutch pick up. Even jumping the normally closed to the plug side will not force the AC Comp clutch to engage, so I think the ground is open between the comp and the high pressure switch. I'd like to find the break in the wire, but I am thinking I'll have to remove the vacuum booster to access the cable, so that's a rainy day project. Everything else works perfectly, fan low, fan high, fan delay, AC fan low, med & high positions.
The only grounds I had to re & re to exchange the booster was the PM & engine strap at the firewall.

 

[salvageV6]

That isn't the same high pressure switch he's talking about.

There's one that snaps into the cavity in the compressor itself on the back of it with two black wires.

Look at the back of the compressor for it, that switch feeds the ground to the compressor clutch.

 

[Agent13]

That isn't the same high pressure switch he's talking about.

There's one that snaps into the cavity in the compressor itself on the back of it with two black wires.

Look at the back of the compressor for it, that switch feeds the ground to the compressor clutch.

That is the problem ! The back pressure switch on the back of the compressor is open. I tested the wires for continuity, ground is there and to the other side of the compressor, just open at the switch. Temporarily jumped the switch and the AC blows cold. My system is still R12 and has a full charge on it with a new dryer and lines to the compressor a few years back. 'll have the switch replaced, but what is the risk of running with the high pressure switch jumped ? Also. when a high pressure switch fails, is it time to consider replacing the compressor ? If the work requires the refrigerant to be removed and the replaced, might be something to just consider replacing the compressor ?

 

[salvageV6]

I think there's a check valve when you replace the switch, but double check that for sure first.

I wouldn't run it without that switch.

They aren't hard to find I don't think.

You need to verify pressures before you can determine for sure the switch is bad.

If you have excessive pressure there at the compressor and it's in place still it's just doing it's job being open.

 

[Agent13]

I think there's a check valve when you replace the switch, but double check that for sure first.

I wouldn't run it without that switch.

They aren't hard to find I don't think.

You need to verify pressures before you can determine for sure the switch is bad.

If you have excessive pressure there at the compressor and it's in place still it's just doing it's job being open.

Will get it checked out. Thanks for the help. I had two issues at the start, AC relay bad, then no ground after replacing the relay, so it's taken a bit to get through the electrical system first. I would think if the switch was okay it would be made when the compressor is off, then open once the compressor starts and hits high pressure for what ever reason, but if there is a check valve, maybe it is stuck leaving the switch pressurized. In any case I'll let the shop take it from here and diagnose whether it is the switch, or there is actually a pressure problem.

Cheers